1. Field of the Invention
The present invention relates to a joint structure of a joint of superconducting wires (or MgB2 wires) using magnesium diboride (MgB2), and a method for joining the MgB2 wires.
2. Description of the Related Art
MgB2 is a superconducting material, having a high critical temperature of 39K and giving high stability of magnetic fields when the material is operated in a persistent mode. Here, disclosed are the following methods for joining MgB2 wires together or joining a MgB2 wire to other superconducting wires such as a NbTi wire and a Nb3Sn wire.
Japanese Laid-Open Patent Publication No. 2006-174546 describes a method for joining MgB2 superconducting wires using superconducting solder. The method for joining superconducting wires using superconducting solder is also used for joining other superconducting wires such as a NbTi wire or the like.
Japanese Laid-Open Patent Publication No. 2003-22719 describes a method for joining MgB2 wires, comprising the steps of: inserting MgB2 wires into a pipe, filling a MgB2 powder thereinto, and compressing the wires and the powder together. Further, a method for mixing a metal having a low melting point is described so as to improve a particle-particle bonding property among MgB2 powder particles.
Further, W. Yao et al. “A Superconducting Joint Technique for MgB2 Round Wires”, IEEE Transaction on Applied Superconductivity, Vol. 19, No. 3, (2009), describes a method comprising the steps of: inserting wires containing a mixed powder of Mg and B powders or MgB2 wire into a cylindrical vessel; filling the mixed powder of Mg and B powders into the vessel from one side of the vessel opposite to the other side of inserting the wires; compressing and heating the above mentioned mixed powder to form MgB2, whereby the MgB2 wires are joined together. Herein, a mixed powder of Mg and B powders forming no MgB2 is filled around the MgB2 wires and subsequently heated. This allows a MgB2 sinter having a good particle-particle bonding property to be produced.
If the superconducting solder described in Japanese Laid-Open Patent Publication No. 2006-174546 is used, superconducting solder can not be used at 10K or more, because the critical temperature of the solder is about 9 k. Therefore, when MgB2 wires are to be joined using superconducting solder, the operational condition at the temperature less than 10K is required, thereby to be unable of making use of a high critical temperature (39K) of MgB2.
When a method using a MgB2 powder described in Japanese laid-open Patent Publication No. 2003-22719 is used, the generated MgB2 powder is filled, which causes a poor particle-particle binding property among MgB2 particles. Accordingly, it is difficult to obtain an electric conduction property required for highly improving the performance of superconducting devices. Hereby, the method has a disadvantage that a time-consuming job is required in order to improve the particle-particle bonding property among the MgB2 particles.
A method described in the document: W. Yao et al “A Superconducting Joint Technique for MgB2 Round Wires”, IEEE Transaction on Applied Superconductivity, Vol. 19, No. 3, (2009), using a mixed powder of Mg and B powders comprises the steps of: filling the mixed powder into the vessel from the opposite side to the wire insertion side, and compressing the above mentioned mixed powder. This causes a drawback that a place in the vicinity of the compressing part becomes the highest density place, while the density decreases as apart from the compressing part. Therefore, if the compressing part is distant from the end parts of the wires, the powder density of the end parts becomes low. In contrast, if the compressing part is set close to the end parts of the wires, the wires may be bended during the compressing process. Accordingly, it is difficult to highly increase the powder density of the Mg and B powders in the vicinity of the end parts of the wires.